YunalescaSummon is right. You just have to make the costume to flatter your body. Everyone has something unique about their body that they have to adjust for everything to fit right. Some people have rounded shoulders, shorter waist, large chest. It's even more imortant for us big girls to cut things to look right on us.
I've always been a firm believer in changing a costume slightly to look actually look better on you. No matter if you are skinny, short, young, old. No one is going to walk up and say "I think her skirt is 1 inch shorter, or his hair curls up, down,and twice in front, or that is not hugging your stomach right." People, unless they are obssessed with all the features of one character, (a few people like that do exist) are just going to notice how good it looks rather than a couple of adjustments. The mind just works that way.
After all, why go through all that trouble to make a really accurate costume when the body type of the character doesn't match yours perfectly and it ends up fitting like you put a potato sack on?
I personally have to add 7 inches to dresses, 3 to skirts, and 4 to pants because of my height. I also leave a little more room for my thighs too. I usually have to take in the sides in at at higher curve because of where my waist is.
Do the job, till it's done Sir